Cell Membranes
(a) outline the roles of membranes within cells and at the surface of cells
(b) state that plasma (cell surface) membranes are partially permeable barriers
Plasma membranes are partially permeable meaning they let some molecules through but not others.
(c) describe, with the aid of diagrams, the fluid mosaic model of membrane structure
The phospholipid bilayer is the basic structural component of plasma membranes. It consists of 2 layers of phospholipid molecules. The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (like ions) through it – it acts as a barrier to these dissolved substances
In the fluid mosaic model, phopholipid molecules form a continuous double layer (bilayer). The bilayer is ‘fluid’ because the phospholipids are constantly moving. The fluid mosaic model also contains cholesterol molecules, proteins, glycoproteins and glycolipids.
(d) describe the roles of components of the cell membrane:
(e) outline the effect of changing temperature on membrane structure and permeability
(f) explain the term cell signalling
(g) explain the role of membrane-bound receptors as sites where hormones and drugs can bind
Cell signalling is when cells communicate with one another by signals. In order to detect signals, cells must have on their surface ‘sensors’ capable of receiving signals, these are known as receptors and are often protein molecules or modified protein molecules (e.g. glycoproteins). In multicellular organisms, communication is often mediated by hormones between cells. Hormones are chemical messengers, produced in specific tissues and then released. Any cell with a receptor for the hormone molecule is called a target cell.
Cells communicate with each other using messenger molecules:
- One cell releases a messenger molecule (e.g. hormone)
- This molecule travels to another cell (e.g. in the blood)
- The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane
Receptor proteins have specific shapes – only messenger molecules with a complementary shape can bind to them. Different cells have different types of receptors – they respond to different messenger molecules. A cell that responds to a particular messenger molecule is called a target cell.
Glycoproteins have receptors. They have a role in:
- cell adhesion – bind cells together in a tissue
- acting as antigens on the surface of cells. Cells of the immune system have receptors that detect the glycoproteins and can determine whether they are ‘self’ or ‘non self’
Many drugs work by binding to receptors in cell membranes. They either trigger a response in the cell, or block the receptor and prevent it from working e.g. cell damage causes the release of histamine. Histamine binds to receptors on the surface of other cells and causes inflammation. Antihistamines work by blocking histamine receptors on cell surfaces. This prevents histamine from binding to the cell and stops inflammation.
(h) explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis
Substances can move across a membrane through 2 processes: passive and active
Active Transport:
(i) explain what is meant by osmosis, in terms of water potential
Osmosis is the movement of water molecules by diffusion from a region of high water potential to a region of low water potential across a partially permeable membrane
Water potential is a measure of the concentration of water molecules that are ‘free’ to diffuse.
Adding solutes to water means the water molecules cluster around the solute molecules, lowering the concentration of ‘free’ water molecules and therefore lowers the water potential.
(j) recognise and explain the effects of solutions of different water potentials can have upon plant and animal cells
In pure water, water moves into a cell by osmosis down a water potential gradient.
- Animal Cell – becomes haemolysed (bursts open)
- Plant Cell – the cell wall prevents bursting. The membrane pushes against the cell wall and the cell becomes turgid
In a very low water potential solution (e.g. concentrated sugar solution), water moves out of a cell by osmosis down a water potential gradient.
- Animal Cell – shrinks and appears wrinkled and the cell becomes crenated
- Plant Cell – the membrane pulls away from the cell wall and the cell becomes plasmolysed